Key
Key turns up in 2 fields: refractories, mechanical. Here is what it means to the people in each one.
Key in refractories
The last brick driven into an arch or ring to lock it tight, or a shape that tapers in width for turning circles.
Two related meanings. In arches and rings, the key is the final brick or bricks driven in at the crown or top to wedge the whole course tight. As a shape, a key brick tapers in width rather than thickness and is used to turn circular linings, as in the key-shaped series used in rotary kilns. Rings are keyed with cut or ground keys, sometimes with steel shims, and a loose key means a loose ring.
"Key that ring tight, if you can push the last brick in by hand it's going to spiral in a week."
Key in mechanical
A solid metal block locks a rotating shaft to a hub to transmit torque.
This precision-machined piece of steel sits half in a slot on a shaft and half in a matching slot in a hub, gear, or pulley. It forces the two components to rotate together while usually allowing them to slide axially during assembly. They come in various profiles, including square, Woodruff, and tapered, and are often designed to shear off before catastrophic overload damages the expensive shaft.
"Make sure you tap the key all the way into the slot before you try to press that drive sprocket onto the motor shaft."